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413 lines
14 KiB
413 lines
14 KiB
From e99c56752a1c4021a93c92b7be78856ebefaa1b3 Mon Sep 17 00:00:00 2001
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From: Stefan Hajnoczi <stefanha@redhat.com>
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Date: Mon, 18 Mar 2024 14:34:29 -0400
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Subject: [PATCH 1/2] coroutine: cap per-thread local pool size
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RH-Author: Stefan Hajnoczi <stefanha@redhat.com>
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RH-MergeRequest: 234: coroutine: cap per-thread local pool size
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RH-Jira: RHEL-28947
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RH-Acked-by: Kevin Wolf <kwolf@redhat.com>
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RH-Acked-by: Hanna Czenczek <hreitz@redhat.com>
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RH-Commit: [1/2] 5971de1c1e238457925bfb9c4bfc932de857b28d (stefanha/centos-stream-qemu-kvm)
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The coroutine pool implementation can hit the Linux vm.max_map_count
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limit, causing QEMU to abort with "failed to allocate memory for stack"
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or "failed to set up stack guard page" during coroutine creation.
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This happens because per-thread pools can grow to tens of thousands of
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coroutines. Each coroutine causes 2 virtual memory areas to be created.
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Eventually vm.max_map_count is reached and memory-related syscalls fail.
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The per-thread pool sizes are non-uniform and depend on past coroutine
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usage in each thread, so it's possible for one thread to have a large
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pool while another thread's pool is empty.
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Switch to a new coroutine pool implementation with a global pool that
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grows to a maximum number of coroutines and per-thread local pools that
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are capped at hardcoded small number of coroutines.
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This approach does not leave large numbers of coroutines pooled in a
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thread that may not use them again. In order to perform well it
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amortizes the cost of global pool accesses by working in batches of
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coroutines instead of individual coroutines.
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The global pool is a list. Threads donate batches of coroutines to when
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they have too many and take batches from when they have too few:
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.-----------------------------------.
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| Batch 1 | Batch 2 | Batch 3 | ... | global_pool
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`-----------------------------------'
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Each thread has up to 2 batches of coroutines:
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.-------------------.
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| Batch 1 | Batch 2 | per-thread local_pool (maximum 2 batches)
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`-------------------'
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The goal of this change is to reduce the excessive number of pooled
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coroutines that cause QEMU to abort when vm.max_map_count is reached
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without losing the performance of an adequately sized coroutine pool.
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Here are virtio-blk disk I/O benchmark results:
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RW BLKSIZE IODEPTH OLD NEW CHANGE
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randread 4k 1 113725 117451 +3.3%
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randread 4k 8 192968 198510 +2.9%
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randread 4k 16 207138 209429 +1.1%
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randread 4k 32 212399 215145 +1.3%
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randread 4k 64 218319 221277 +1.4%
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randread 128k 1 17587 17535 -0.3%
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randread 128k 8 17614 17616 +0.0%
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randread 128k 16 17608 17609 +0.0%
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randread 128k 32 17552 17553 +0.0%
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randread 128k 64 17484 17484 +0.0%
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See files/{fio.sh,test.xml.j2} for the benchmark configuration:
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https://gitlab.com/stefanha/virt-playbooks/-/tree/coroutine-pool-fix-sizing
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Buglink: https://issues.redhat.com/browse/RHEL-28947
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Reported-by: Sanjay Rao <srao@redhat.com>
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Reported-by: Boaz Ben Shabat <bbenshab@redhat.com>
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Reported-by: Joe Mario <jmario@redhat.com>
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Reviewed-by: Kevin Wolf <kwolf@redhat.com>
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Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
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Message-ID: <20240318183429.1039340-1-stefanha@redhat.com>
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(cherry picked from commit 86a637e48104ae74d8be53bed6441ce32be33433)
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Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
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---
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util/qemu-coroutine.c | 282 +++++++++++++++++++++++++++++++++---------
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1 file changed, 223 insertions(+), 59 deletions(-)
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diff --git a/util/qemu-coroutine.c b/util/qemu-coroutine.c
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index 5fd2dbaf8b..2790959eaf 100644
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--- a/util/qemu-coroutine.c
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+++ b/util/qemu-coroutine.c
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@@ -18,39 +18,200 @@
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#include "qemu/atomic.h"
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#include "qemu/coroutine_int.h"
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#include "qemu/coroutine-tls.h"
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+#include "qemu/cutils.h"
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#include "block/aio.h"
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-/**
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- * The minimal batch size is always 64, coroutines from the release_pool are
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- * reused as soon as there are 64 coroutines in it. The maximum pool size starts
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- * with 64 and is increased on demand so that coroutines are not deleted even if
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- * they are not immediately reused.
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- */
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enum {
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- POOL_MIN_BATCH_SIZE = 64,
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- POOL_INITIAL_MAX_SIZE = 64,
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+ COROUTINE_POOL_BATCH_MAX_SIZE = 128,
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};
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-/** Free list to speed up creation */
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-static QSLIST_HEAD(, Coroutine) release_pool = QSLIST_HEAD_INITIALIZER(pool);
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-static unsigned int pool_max_size = POOL_INITIAL_MAX_SIZE;
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-static unsigned int release_pool_size;
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+/*
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+ * Coroutine creation and deletion is expensive so a pool of unused coroutines
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+ * is kept as a cache. When the pool has coroutines available, they are
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+ * recycled instead of creating new ones from scratch. Coroutines are added to
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+ * the pool upon termination.
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+ *
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+ * The pool is global but each thread maintains a small local pool to avoid
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+ * global pool contention. Threads fetch and return batches of coroutines from
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+ * the global pool to maintain their local pool. The local pool holds up to two
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+ * batches whereas the maximum size of the global pool is controlled by the
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+ * qemu_coroutine_inc_pool_size() API.
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+ *
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+ * .-----------------------------------.
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+ * | Batch 1 | Batch 2 | Batch 3 | ... | global_pool
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+ * `-----------------------------------'
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+ *
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+ * .-------------------.
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+ * | Batch 1 | Batch 2 | per-thread local_pool (maximum 2 batches)
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+ * `-------------------'
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+ */
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+typedef struct CoroutinePoolBatch {
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+ /* Batches are kept in a list */
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+ QSLIST_ENTRY(CoroutinePoolBatch) next;
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+
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+ /* This batch holds up to @COROUTINE_POOL_BATCH_MAX_SIZE coroutines */
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+ QSLIST_HEAD(, Coroutine) list;
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+ unsigned int size;
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+} CoroutinePoolBatch;
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+
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+typedef QSLIST_HEAD(, CoroutinePoolBatch) CoroutinePool;
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+
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+/* Host operating system limit on number of pooled coroutines */
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+static unsigned int global_pool_hard_max_size;
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+
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+static QemuMutex global_pool_lock; /* protects the following variables */
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+static CoroutinePool global_pool = QSLIST_HEAD_INITIALIZER(global_pool);
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+static unsigned int global_pool_size;
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+static unsigned int global_pool_max_size = COROUTINE_POOL_BATCH_MAX_SIZE;
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+
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+QEMU_DEFINE_STATIC_CO_TLS(CoroutinePool, local_pool);
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+QEMU_DEFINE_STATIC_CO_TLS(Notifier, local_pool_cleanup_notifier);
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-typedef QSLIST_HEAD(, Coroutine) CoroutineQSList;
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-QEMU_DEFINE_STATIC_CO_TLS(CoroutineQSList, alloc_pool);
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-QEMU_DEFINE_STATIC_CO_TLS(unsigned int, alloc_pool_size);
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-QEMU_DEFINE_STATIC_CO_TLS(Notifier, coroutine_pool_cleanup_notifier);
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+static CoroutinePoolBatch *coroutine_pool_batch_new(void)
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+{
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+ CoroutinePoolBatch *batch = g_new(CoroutinePoolBatch, 1);
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+
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+ QSLIST_INIT(&batch->list);
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+ batch->size = 0;
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+ return batch;
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+}
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-static void coroutine_pool_cleanup(Notifier *n, void *value)
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+static void coroutine_pool_batch_delete(CoroutinePoolBatch *batch)
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{
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Coroutine *co;
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Coroutine *tmp;
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- CoroutineQSList *alloc_pool = get_ptr_alloc_pool();
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- QSLIST_FOREACH_SAFE(co, alloc_pool, pool_next, tmp) {
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- QSLIST_REMOVE_HEAD(alloc_pool, pool_next);
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+ QSLIST_FOREACH_SAFE(co, &batch->list, pool_next, tmp) {
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+ QSLIST_REMOVE_HEAD(&batch->list, pool_next);
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qemu_coroutine_delete(co);
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}
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+ g_free(batch);
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+}
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+
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+static void local_pool_cleanup(Notifier *n, void *value)
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+{
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+ CoroutinePool *local_pool = get_ptr_local_pool();
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+ CoroutinePoolBatch *batch;
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+ CoroutinePoolBatch *tmp;
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+
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+ QSLIST_FOREACH_SAFE(batch, local_pool, next, tmp) {
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+ QSLIST_REMOVE_HEAD(local_pool, next);
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+ coroutine_pool_batch_delete(batch);
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+ }
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+}
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+
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+/* Ensure the atexit notifier is registered */
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+static void local_pool_cleanup_init_once(void)
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+{
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+ Notifier *notifier = get_ptr_local_pool_cleanup_notifier();
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+ if (!notifier->notify) {
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+ notifier->notify = local_pool_cleanup;
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+ qemu_thread_atexit_add(notifier);
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+ }
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+}
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+
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+/* Helper to get the next unused coroutine from the local pool */
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+static Coroutine *coroutine_pool_get_local(void)
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+{
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+ CoroutinePool *local_pool = get_ptr_local_pool();
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+ CoroutinePoolBatch *batch = QSLIST_FIRST(local_pool);
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+ Coroutine *co;
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+
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+ if (unlikely(!batch)) {
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+ return NULL;
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+ }
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+
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+ co = QSLIST_FIRST(&batch->list);
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+ QSLIST_REMOVE_HEAD(&batch->list, pool_next);
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+ batch->size--;
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+
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+ if (batch->size == 0) {
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+ QSLIST_REMOVE_HEAD(local_pool, next);
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+ coroutine_pool_batch_delete(batch);
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+ }
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+ return co;
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+}
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+
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+/* Get the next batch from the global pool */
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+static void coroutine_pool_refill_local(void)
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+{
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+ CoroutinePool *local_pool = get_ptr_local_pool();
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+ CoroutinePoolBatch *batch;
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+
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+ WITH_QEMU_LOCK_GUARD(&global_pool_lock) {
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+ batch = QSLIST_FIRST(&global_pool);
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+
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+ if (batch) {
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+ QSLIST_REMOVE_HEAD(&global_pool, next);
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+ global_pool_size -= batch->size;
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+ }
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+ }
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+
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+ if (batch) {
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+ QSLIST_INSERT_HEAD(local_pool, batch, next);
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+ local_pool_cleanup_init_once();
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+ }
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+}
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+
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+/* Add a batch of coroutines to the global pool */
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+static void coroutine_pool_put_global(CoroutinePoolBatch *batch)
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+{
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+ WITH_QEMU_LOCK_GUARD(&global_pool_lock) {
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+ unsigned int max = MIN(global_pool_max_size,
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+ global_pool_hard_max_size);
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+
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+ if (global_pool_size < max) {
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+ QSLIST_INSERT_HEAD(&global_pool, batch, next);
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+
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+ /* Overshooting the max pool size is allowed */
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+ global_pool_size += batch->size;
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+ return;
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+ }
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+ }
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+
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+ /* The global pool was full, so throw away this batch */
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+ coroutine_pool_batch_delete(batch);
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+}
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+
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+/* Get the next unused coroutine from the pool or return NULL */
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+static Coroutine *coroutine_pool_get(void)
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+{
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+ Coroutine *co;
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+
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+ co = coroutine_pool_get_local();
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+ if (!co) {
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+ coroutine_pool_refill_local();
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+ co = coroutine_pool_get_local();
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+ }
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+ return co;
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+}
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+
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+static void coroutine_pool_put(Coroutine *co)
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+{
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+ CoroutinePool *local_pool = get_ptr_local_pool();
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+ CoroutinePoolBatch *batch = QSLIST_FIRST(local_pool);
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+
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+ if (unlikely(!batch)) {
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+ batch = coroutine_pool_batch_new();
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+ QSLIST_INSERT_HEAD(local_pool, batch, next);
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+ local_pool_cleanup_init_once();
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+ }
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+
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+ if (unlikely(batch->size >= COROUTINE_POOL_BATCH_MAX_SIZE)) {
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+ CoroutinePoolBatch *next = QSLIST_NEXT(batch, next);
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+
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+ /* Is the local pool full? */
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+ if (next) {
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+ QSLIST_REMOVE_HEAD(local_pool, next);
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+ coroutine_pool_put_global(batch);
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+ }
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+
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+ batch = coroutine_pool_batch_new();
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+ QSLIST_INSERT_HEAD(local_pool, batch, next);
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+ }
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+
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+ QSLIST_INSERT_HEAD(&batch->list, co, pool_next);
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+ batch->size++;
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}
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Coroutine *qemu_coroutine_create(CoroutineEntry *entry, void *opaque)
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@@ -58,31 +219,7 @@ Coroutine *qemu_coroutine_create(CoroutineEntry *entry, void *opaque)
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Coroutine *co = NULL;
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if (IS_ENABLED(CONFIG_COROUTINE_POOL)) {
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- CoroutineQSList *alloc_pool = get_ptr_alloc_pool();
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-
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- co = QSLIST_FIRST(alloc_pool);
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- if (!co) {
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- if (release_pool_size > POOL_MIN_BATCH_SIZE) {
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- /* Slow path; a good place to register the destructor, too. */
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- Notifier *notifier = get_ptr_coroutine_pool_cleanup_notifier();
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- if (!notifier->notify) {
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- notifier->notify = coroutine_pool_cleanup;
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- qemu_thread_atexit_add(notifier);
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- }
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-
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- /* This is not exact; there could be a little skew between
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- * release_pool_size and the actual size of release_pool. But
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- * it is just a heuristic, it does not need to be perfect.
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- */
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- set_alloc_pool_size(qatomic_xchg(&release_pool_size, 0));
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- QSLIST_MOVE_ATOMIC(alloc_pool, &release_pool);
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- co = QSLIST_FIRST(alloc_pool);
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- }
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- }
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- if (co) {
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- QSLIST_REMOVE_HEAD(alloc_pool, pool_next);
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- set_alloc_pool_size(get_alloc_pool_size() - 1);
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- }
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+ co = coroutine_pool_get();
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}
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if (!co) {
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@@ -100,19 +237,10 @@ static void coroutine_delete(Coroutine *co)
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co->caller = NULL;
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if (IS_ENABLED(CONFIG_COROUTINE_POOL)) {
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- if (release_pool_size < qatomic_read(&pool_max_size) * 2) {
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- QSLIST_INSERT_HEAD_ATOMIC(&release_pool, co, pool_next);
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- qatomic_inc(&release_pool_size);
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- return;
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- }
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- if (get_alloc_pool_size() < qatomic_read(&pool_max_size)) {
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- QSLIST_INSERT_HEAD(get_ptr_alloc_pool(), co, pool_next);
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- set_alloc_pool_size(get_alloc_pool_size() + 1);
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- return;
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- }
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+ coroutine_pool_put(co);
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+ } else {
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+ qemu_coroutine_delete(co);
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}
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-
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- qemu_coroutine_delete(co);
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}
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void qemu_aio_coroutine_enter(AioContext *ctx, Coroutine *co)
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@@ -223,10 +351,46 @@ AioContext *qemu_coroutine_get_aio_context(Coroutine *co)
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void qemu_coroutine_inc_pool_size(unsigned int additional_pool_size)
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{
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- qatomic_add(&pool_max_size, additional_pool_size);
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+ QEMU_LOCK_GUARD(&global_pool_lock);
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+ global_pool_max_size += additional_pool_size;
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}
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void qemu_coroutine_dec_pool_size(unsigned int removing_pool_size)
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{
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- qatomic_sub(&pool_max_size, removing_pool_size);
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+ QEMU_LOCK_GUARD(&global_pool_lock);
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+ global_pool_max_size -= removing_pool_size;
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+}
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+
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+static unsigned int get_global_pool_hard_max_size(void)
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+{
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+#ifdef __linux__
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+ g_autofree char *contents = NULL;
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+ int max_map_count;
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+
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+ /*
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+ * Linux processes can have up to max_map_count virtual memory areas
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+ * (VMAs). mmap(2), mprotect(2), etc fail with ENOMEM beyond this limit. We
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+ * must limit the coroutine pool to a safe size to avoid running out of
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+ * VMAs.
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+ */
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+ if (g_file_get_contents("/proc/sys/vm/max_map_count", &contents, NULL,
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+ NULL) &&
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+ qemu_strtoi(contents, NULL, 10, &max_map_count) == 0) {
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+ /*
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+ * This is a conservative upper bound that avoids exceeding
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+ * max_map_count. Leave half for non-coroutine users like library
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+ * dependencies, vhost-user, etc. Each coroutine takes up 2 VMAs so
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+ * halve the amount again.
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+ */
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+ return max_map_count / 4;
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+ }
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+#endif
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+
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+ return UINT_MAX;
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+}
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+
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+static void __attribute__((constructor)) qemu_coroutine_init(void)
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+{
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+ qemu_mutex_init(&global_pool_lock);
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+ global_pool_hard_max_size = get_global_pool_hard_max_size();
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}
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--
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2.39.3
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